Plasmon-Exciton-Polaritons from Nanoparticle Lattices Strongly Coupled to Metal–Organic Framework Ligands

Strong coupling between light and matter has received significant attention for its interesting fundamental properties and diverse applications. Despite the breadth of work done in this field, the systems capable of achieving strong coupling are still relatively limited. To continue to diversify polariton work, new excitonic materials and cavities must be developed. Surface-mounted metal-organic frameworks (MOF) with a porphyrin-based ligand were synthesized on top of Ag nanoparticle lattices. Angle-resolved transmission measurements of the coupled system reveal a lower energy polariton mode with signature avoided crossing at the exciton energy. With modeling, the upper polariton was predicted and estimated to have a Rabi splitting of 110 meV. By changing the refractive index of the solvent in the MOF, the plasmon energy could be tuned and thus the coupling strength could be controlled from the strong to weak regime. Transient absorption spectroscopy showed that the lower polariton decays rapidly at <500 ps scales, but at later times scales, the decay is slowed by energy transfer from the upper polariton. This work highlights MOFs as an effective molecular exciton and plasmonic lattices as an accessible open cavity for strong light-matter coupling.